The present invention relates to a magnetic device for suppressing surge current generated by a stray capacitance between a high-voltage DC power supply and a load (ion source) at the time of discharge breakdown of the ion source in a neutral beam injector for a nuclear fusion reactor, etc. and to a surge current-suppressing circuit comprising such a magnetic device.
A neutral beam injector comprises an accelerator for accelerating ions in a plasma gas, neutralizing them and injecting them into another plasma, etc. In this neutral beam injector, the accelerator should have a function to suppress current flowing as a surge from a high-voltage DC power supply side to an ion source at the time of discharge breakdown of the ion source which is a load in this apparatus, thereby preventing the ion source from being destroyed and also preventing continuous discharge breakdown.
Specifically, the accelerator is constituted by a circuit as shown in FIG. 1. When discharge breakdown is caused in the ion source 4, a high-output tetrode 42 (hereinafter referred to as "regulator tube") provided between an unstable high-voltage DC power supply 41 and an ion source 4 for conducting regulation is cut off in about 20 .mu.s or less, thereby preventing energy from flowing from the high-voltage DC power supply 41 to the ion source 4. In addition, to absorb energy stored in a stray capacitance 1 hypothetically existing on the load side from the regulator tube 42, a surge current-suppressing circuit, which is called "surge blocker, shown by the block "A" in the figure is provided. The surge blocker "A" comprises a magnetic core 2 constituted by toroidal ferrite, etc., a primary winding 3, and a secondary winding 8 provided with a resistor 9 for suppressing the ringing of discharge current i.sub.d supplied from the stray capacitance 1 at the time of short-circuiting of the ion source 4 and for consuming the energy of the discharge current i.sub.d.
Incidentally, in FIG. 1, 5 denotes a wire inductance (stray inductance generated by wiring, and 6 denotes a switch depicted as a model for showing a short-circuiting phenomenon of the ion source 4. When the switch 6 is turned on, the ion source 4 is short-circuited. 7 denotes a voltage source for showing voltage decrease at the time of short-circuiting of the ion source 4.
FIG. 2 schematically shows an operating B-H hysteresis loop of the magnetic core 2 after the short-circuiting of the ion source 4 in the circuit of FIG. 1.
The operation of the surge current-suppressing circuit for the high-voltage DC power supply will be explained referring to FIGS. 1 and 2. First, before the short-circuiting of the ion source 4, namely in the normal operation state, an operating point of the magnetic core 2 is at a magnetic flux density B.sub.B ' determined by DC magnetization H.sub.B ' generated by DC current I.sub.B ' stationally flowing from the high-voltage DC power supply 41 via the regulator tube 42.
Here, H.sub.B ' is determined by the following equation: ##EQU1## N.sub.p : Number of winding in the primary winding. Mean magnetic path length (m) of the magnetic core.
When the ion source 4 is short-circuited, namely when the switch 6 is turned on, the regulator tube 42 is cut off so that the current I.sub.B ' supplied from the high-voltage DC power supply 41 is prevented from flowing into the ion source 4. However, by electric charge stored in the stray capacitance 1, discharge current i.sub.d is caused to flow into the ion source 4 in a time .tau.. This current is called "surge current". The surge blocker "A" prevents the surge current from flowing into the ion source 4. The magnetic core 2 in the surge blocker A serves to prevent the flow of the surge current to the extent shown by a voltage.time product determined by the following equation: ##EQU2## v.sub.p : Voltage in the primary winding, Ae: Effective cross section (m.sup.2) of the magnetic core, and
.DELTA.B: Operating magnetic flux density range,
and suppress the surge current to a current wave height expressed by the following formula: ##EQU3## which is determined by the maximum magnetization H.sub.m : ##EQU4## H.sub.B ': DC magnetization corresponding to B.sub.B ', and .mu..sub.rp : Relative pulse permeability.
The above maximum magnetization H.sub.m corresponds to the maximum magnetic flux density: EQU B.sub.m =.DELTA.B+B.sub.B '(T). (5)
Based on the above principle, to keep the surge current I.sub.D equal to or lower than a permissible level I.sub.Dp which depends upon the circuit, the following formulae (6) and (7) should be satisfied: ##EQU5##
The formula (6) is derived from the formulae (2) and (5), in which .DELTA.B represents an operating magnetic flux density range, and B.sub.mp represents a maximum magnetic flux density at which the magnetic core is operable. The formula (7) is derived from the formulae (1), (3) and (4), in which .mu..sub.rp, represents an effective relative pulse permeability. With respect to the effective relative pulse permeability .mu..sub.rp please refer to JIS Handbook, Electronics, "Test Methods of Coil and Transformer Ferrite Magnetic Cores C2562, published by the Japan Standard Association.
In the design of an actual surge blocker, the voltage-time product, I.sub.B ' and B.sub.B ' are determined by the specifications of a neutral beam injector. In addition, for the purpose of miniaturization of the magnetic cores at N.sub.p =1 turn, B.sub.mp should be as large as possible. However, B.sub.mp is inherently restricted by the materials of the magnetic cores. B.sub.B ' and .mu..sub.rp are also values determined by the magnetic core materials. Accordingly, appropriate magnetic core materials should be selected.
To achieve the magnetic core design satisfying both formulae (6) and (7), it is important to select proper materials and determine appropriate Ae and le.
As is clear from the formula (6), the larger (B.sub.mp -B.sub.B '), the smaller Ae, enabling the miniaturization of the magnetic core.
FIG. 3 shows an improvement in which the operating magnetic flux density range .DELTA.B is increased in the system shown in FIG. 1. In FIG. 3, 10 denotes a bias DC power supply, and 11 and 12 a capacitor and an inductance for suppressing surge voltage induced in the secondary winding at the time of short-circuiting of the ion source. In this circuit, since the bias current I.sub.B can be caused to flow in the direction shown by the arrow, the operating magnetic flux density range .DELTA.B can be increased as shown in FIG. 4, enabling the further miniaturization of the magnetic core 2. Incidentally, in FIG. 4, ##EQU6## N.sub.S : Number of winding in the secondary winding. B.sub.B represents a magnetic flux density at H.sub.B determined by the above formula. Incidentally, with respect to the details of the accelerator for the neutral beam injector, please refer to, for instance, Watanabe et al., "Composition of the Power Supply System of JT-60 Active Beam Source and Suppression of Surge Current, "The Japan Atomic Energy Research Institute, JAERI-M 86-104, July 1986.
In the surge current-suppressing magnetic device in the above accelerator, a neutral beam accelerator having relatively low acceleration voltage of 100 keV and 100 A is used because the nuclear fusion reactor itself is still in the research stage. At present, the magnetic core is made of Ni-Zn ferrite showing as low a saturation magnetic flux density as about 0.35 T, and a large number of Ni-Zn ferrite magnetic cores of about 500 mm in diameter are stacked. However, a neutral beam injector to be used in the future experimental nuclear fusion reactor is expected to be operated at a high acceleration voltage of 1 MeV and 50 A. In this case, if the surge blocker is constituted by stacked magnetic cores of Ni-Zn ferrite, the total length of the stacked magnetic cores would become more than 20 m. Accordingly, from the aspect of weight and space, it is totally impractical to use Ni-Zn ferrite magnetic cores.
Alternatively, to minimize the magnetic core size, it is possible to increase .DELTA.B by providing a bias circuit shown in FIG. 3. However, since the Ni-Zn ferrite shows a squareness ratio of 0.6 or less in a DC magnetic curve and a coercive force of several tens A/m or more, bias current I.sub.B is inevitably large. Accordingly, the bias circuit is made large, thereby restricting a practically operating magnetic flux density range to 0.3 T or so. Thus, this is not a promising way for the reduction of the magnetic core size.
With respect to the magnetic core materials, amorphous alloy materials showing higher permeabilities (described in Japanese Patent Publication No. 55-19976) and iron-base fine crystalline alloys showing higher permeabilities and higher saturation magnetic flux densities (described in EP 87114568.6) than crystalline alloys are known. However, they have never been used for magnetic devices for suppressing surge current in the accelerator for the neutral beam injector. Thus, their actual operations have been totally unpredictable.